<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .one{
            width:500px;
            height:400px;
            border:1px solid red;
            position:relative;
        }
        .two{
            width:300px;
            height:200px;
            border:1px solid blue;
            position:absolute;
            margin-left:250px;
            left:-150px;                     /*浏览器中的坐标，上负，下正，左负，右正*/
        }
    </style>
</head>
<body>
   <div class="one">
       <div class="two"></div>
   </div>
</body>
</html>